This invention generally relates to turbine buckets. More specifically, the invention relates to a method for determining conditional based maintenance of first stage turbine buckets.
A gas turbine engine is an air-dependent device that uses exhaust-gas-driven turbine wheels to drive a compressor, making continuous operation of the engine possible. Turbines extract kinetic energy from the expanding gases as the gases exit from the burners. The turbines convert this energy into shaft horsepower to drive the compressor and engine accessories. Turbines typically used for power generation are comprised of multiple stages, each stage having fixed partitions and a plurality of turbine buckets, also known as blades, mounted on rotatable turbine wheels.
Turbines are subjected to high speeds and high temperatures. These high speeds result in high centrifugal forces. Turbines must operate close to temperature limits that, if exceeded, lower the strength of the materials used to form the turbine components. Turbine buckets undergo distortion or lengthening known as xe2x80x9ccreep.xe2x80x9d Creep, which is a cumulative condition, refers to stretching or elongation of the bucket. The rate of creep is determined by the load imposed on the turbine, and the strength of the bucket. The strength of the bucket is in turn determined by the temperature within the turbine. Since changes in pitch and creep are more pronounced if engine operating limits are not respected, the temperature and RPM limits stipulated by the manufacturer must be followed.
The turbine assembly is made of two main parts: the disc or wheel, and the buckets or blades. The disc or wheel is a statically and dynamically balanced unit of specially alloyed steel. It usually contains large percentages of chromium, nickel, and cobalt. After forging, the entire surface of the disc is machined and carefully inspected using X-rays, magnetism, and other inspection methods to ensure its structural integrity. The buckets are forged from highly alloyed steel, and are carefully machined and inspected before being certified for use. The buckets are attached to the disc by means of a fir tree design to allow for different rates of expansion between the disc and the buckets, while still holding the bucket firmly against centrifugal loads. The bucket is kept from moving axially either by rivets, special locking tabs or devices, or another turbine stage.
It is possible that oxidation of turbine buckets may occur through normal usage. This oxidation may lead to decreased bucket life and subsequent failure of the turbines. First stage turbine bucket leading edge oxidation reflects local overheating that can cause early coating depletion and can cause base metal attack. Since welding in the airfoil section is prohibited, these processes can lead to early retirement of buckets if not discovered before base metal attack occurs. Experience has shown that first stage turbine buckets may develop leading edge oxidation at the first hot gas path interval inspection. However, a large variability exists in the amount of oxidation that may develop between buckets in a given unit. Some buckets having oxidation only require stripping and recoating, while other buckets are irreparable and must be replaced with new buckets.
Current methods used to determine bucket failure are not ideal. Generally, turbine buckets are inspected at specified intervals. These intervals are based on a combination of fired starts and fired hours. Inspection requires stopping the turbine and determining if the bucket needs to be refurbished or even replaced. This method of maintenance has become known as interval based maintenance.
There exists a need in the art for a method to predict if a bucket requires servicing. This method should provide for monitoring of the bucket without inspection, while the turbine is in operation. The method should also provide a better understanding of the impact of local temperature uncertainties. A better understanding of temperature uncertainties would lead to the ability to predict bucket failure.
In accordance with certain preferred aspects, a method is provided for conditional based maintenance of first stage turbine buckets. The method allows for monitoring of the buckets during operation of the turbine. The method comprises monitoring one or more parameters that are indicative of bucket failure. These parameters may include temperature, oxidation of the buckets, reduction in power output, irregular operation, or other parameters indicative of turbine and turbine bucket failure. Preferably, oxidation of the buckets is indirectly monitored. More preferably, oxidation of the buckets is indirectly monitored by monitoring the temperature of the turbine buckets to determine the condition of the buckets. Most preferably, the temperature of a turbine bucket is measured and compared with the temperature of a reference turbine bucket that has no oxidation. The comparison of the temperature of the turbine bucket with the temperature of the reference bucket is known as relative temperature measurement.
As used herein, conditional based maintenance means maintenance of a component based on the condition of the component. Therefore, unlike the interval based maintenance discussed above, conditional based maintenance occurs when one or more parameters indicate that the turbine bucket requires maintenance. Conditional based maintenance offers several advantages over interval based maintenance. For example, often during interval based maintenance, it is found that the turbine components do not require maintenance and, therefore, the turbine was shut down unnecessarily. Conditional based maintenance prevents unnecessary shut down of the turbine and, thus, provides increased operating time for the turbine.
As used herein, the term relative temperature measurement means that the temperature of a turbine bucket is measured and compared with a reference turbine bucket, which has no oxidation. The bucket having no oxidation acts as a control, baseline, or reference for all measurements. The baseline also normalizes measurements that are made on different days.
In accordance with preferred embodiments, an analytical procedure that utilizes a temperature sensor to calculate relative temperature data is described herein. More preferably, optical pyrometer temperature data is used to calculate relative temperature data. The relative temperature data is used as an indicator of the initiation and spread of oxidation on the leading edge of first stage turbine buckets, and increases operating time of the turbine.
Advantages of this invention include a method for determining when a bucket needs servicing based on an analysis of data obtained while the turbine is operating, instead of using interval based servicing, which requires shutdown of the turbine. Using this method can reduce the number of buckets that are irreparable as a result of extensive oxidation, and increase operating time of the turbine.